In recording or sensing data on an optical storage disk, accurate focus of the impinging light beam is critical. A real time servo system is needed to keep the optics in focus. A laser supplies a collimated monochromatic light beam which is focused by a lens system onto a rotating optical disk. The disk reflects the beam through a lens system to a beamsplitter which directs a portion of this beam to a focus detection system. A focus error signal is generated and electronically coupled to the focussing lens system. Most focus detection techniques rely on the following properties of the returning beam. If the disk is at the focal plane of the focussing lens system and the light source is collimated, the return beam will also be collimated. If the disk is too close to the lens system the return beam will be more divergent and if it is too far the return beam will be more convergent.
The astigmatic focus scheme is representative of the conventional focus detection technology. A cylindrical lens provides different focal lengths for the horizontal and vertical axes of the return light beam. A quadrant detector placed at the average of the focal lengths senses a round pattern when the beam is in focus and vertically or horizontally elliptical patterns when the focus is too-close or too-far. A disadvantage of this method is the difficulty in aligning the cylindrical lens and quadrant detector along the light propagation axis and the two orthogonal axes, and in aligning the angular orientation of the detector about the light propagation axis. Further, this method is sensitive to environmental influences such as mechanical shock and temperature variations. Thermal shifts in the refractive index of optical components and in the optical path length change the focus of the astigmatic lens on the detector and thus degrade the focus detection accuracy.
Numerous focus detection schemes have been proposed which are less sensitive to thermal shifts (see, for example, Ohara et al. in U.S. Pat. No. 4,724,533 and Smid et al. in U.S. Pat. No. 4,712,205). However, they typically function by focussing one or more beams onto a plurality of detectors and thus have the critical alignment requirements of the astigmatic system. D. K. Cohen (U.S. Pat. No. 4,604,739 and Ph.D. Thesis, Univ. of Arizona, 1987) disclosed a method of focus detection employing rotated interference patterns. The technique relies on the properties of lateral shearing interferometers (Malacara in Optical Shop Testing, John Wiley & Sons, New York, 1978, pp. 105-148). If a collimated beam of light impinges on a lateral shearing interferometer (LSI) an interference pattern is produced. If the beam is rendered diverging, as when the optical disk is too close to the focus, the interference pattern rotates in one direction and if the beam is converging the interference pattern rotates in the opposite direction. A quadrant detector placed in the interference pattern detects the rotational orientation of the interference fringes. With the LSI of Cohen's invention, the interference pattern does not change with propagation so the quadrant detector can be placed at any distance from the interferometer. This provides design flexibility and permits a compact detection device. In the directions orthogonal to the light propagation axis the quadrant detector must be centered on an interference fringe, a far less stringent requirement than placement at the focal point of a lens. In addition, this technique reduces sensitivity to the exact optical path lengths and thus to temperature variations.
Cohen's patent does not address a critical aspect of the lateral shearing interferometer used in this application. Diode lasers are, in general, the light source of preference in optical storage systems. Typically, they have short coherence lengths of less than 1 mm. Consequently no fringes will be visible at the quadrant detector when the path length difference between the beams that form the interference pattern exceeds this coherence length. It is therefore an object of the present invention to solve this important implementation issue with the introduction of a compact equal path length lateral shearing interferometer.